1. Field of the Invention
The present invention relates generally to conductive, silicone-based compositions, with improved intial adhesion and reduced micro-voiding.
2. Brief Description of Related Technology
Advances in the electronic industry have made thermal management an increasingly important consideration, particularly with respect to packaging issues. For instance, heat build-up in electronic products leads to reduced reliability (“mean-time-to-failure”), slower performance, and reduced power-handling capabilities. In addition, continued interest in increasing the number of electronic components on, while reducing the size of, semiconductor chips, notwithstanding the desire generally to reduce power consumption thereof, also contributes to the importance of thermal management. Also, chip-on-board technology, where semiconductor chips are mounted directly to printed circuit boards, creates further demands on thermal management because of the more efficient use of surface area thereon. Thus, it is not surprising that packaging technology has been called one of the greatest single factors limiting the electronics industry. See M. M. Konarski and J. Heaton, “Electronic Packaging Design Advances Miniaturization”, Circ. Assembly, 32-35 (August 1996).
A heat sink, constructed from a lightweight thermally conductive material, such as aluminum alloy or graphite composite, is often used with electronic devices to facilitate heat dissipation therefrom.
Interfacial thermal resistance between the heat sink and the heat-generating electronic device has however presented obstacles in effeciently dissipating heat as intended. Generally, such resistance may be minimized by positioning at the interface junction a material having (1) high thermal conductivity, (2) intimate surface contact with the heat sink and electronic device, and (3) good durability, such as is measured by thermal cycling which detects failure or performance loss at the interface junction between the heat sink and the heat-generating device. Thermally conductive greases, mica chips and ceramic insulators, pads and tapes, and adhesives have been used as such interface materials.
Surface contact between the heat sink and the electronic device may be improved using a variety of materials, such as a thermal grease (which penetrates such interfacial or surface voids, thereby effectively lowering interfacial thermal resistance), mica chips (while inexpensive with excellent dielectric strength; they are brittle and easily damaged), thermally conductive pads [laminated composite materials, which are often coated with pressure-sensitive adhesives to facilitate bonding and good thermal contact with the substrate surfaces between which they are positioned, see e.g., U.S. Pat. No. 4,574,879 (DeGree)], and thermally conductive adhesives (which are curable), as contrasted to greases (which are not intended to be curable).
Various thermally conductive adhesives (such as those based on silicone, epoxy, phenolic, vinyl, and/or acrylic materials) are known for use in a number of applications, such as sealants, fuser roll coatings in electrostatic copying machines, bonding media, and the like.
Thermal conductivity improvments of such adhesives are desirable, and may often be attained by the addition of a conductive filler to the resin matrix. [See Handbook of Fillers for Plastics, 6.1, 255, H. S. Katz and J. V. Milewski, eds., Van Nostrand Reinhold Co., New York (1987); see also U.S. Pat. No. 4,147,669 (Shaheen) (gallium, aluminum, and gold, copper or silver in a resin); U.S. Pat. No. 4,544,696 (Streusand), U.S. Pat. No. 4,584,336 (Pate) and U.S. Pat. No. 4,588,768 (Streusand) (silicon nitride-containing organopolysiloxane with aluminum oxide or zinc oxide); U.S. Pat. No. 5,011,870 (Peterson) (aluminum nitride, and silicon metal and boron nitride in a polyorganosilicone resin matrix); and U.S. Pat. No. 5,352,731 (Nakano) (aluminum oxide-containing silicone rubber).]
U.S. Pat. No. 5,430,085 (Acevedo) describes a thermally and electrically conductive caulk including a resin, such as silicone, mixed with a filler which includes 80% by weight conductive particles with a particle size in the range of 300 to 325 microns, 10% by weight conductive particles with a particle size in the range of 75 to 80 microns, and 10% by weight conductive fibers having a length in the range of 0.020 to 0.025 inches.
U.S. Pat. No. 4,604,424 (Cole) describes thermally conductive silicone elastomers containing a polydiorganosiloxane, a curing agent, a platinum-containing hydrosilation catalyst, and zinc oxide and magnesium oxide fillers, the particle size of which fillers is such that substantially all of the filler particles pass through a 325 mesh screen, and the average particle size of which fillers is below 10 microns. The filler is composed of 50% to 90% zinc oxide, and 10% to 50% magnesium oxide, each by weight of the filler. Other fillers (up to 40% by weight) include aluminum oxide, ferric oxide and carbon black. The cured elastomers are said to resist erosion by abrasive materials to a greater extent than compositions containing aluminum oxide as the sole filler.
U.S. Pat. No. 4,444,944 (Matsushita) speaks to thermally conductive silicone rubber composition of 100 parts by weight of a polyorganosiloxane having a viscosity at 25° C. of from 0.1 to 100 Pa·s and having a certain average unit formula, a polyorganohydrogensiloxane having a viscosity at 25° C. of 0.0007 to 5 Pa·s and having a certain average unit formula, from 100 to 500 parts by weight of alumina powder having an average particle size of 2 microns to 10 microns and an oil absorption of ≧15 mL/g, and a platinum catalyst.
In U.S. Pat. No. 5,445,308 (Nelson), another method of improving thermal conductivity provides a connection between spaced surfaces by mixing a thermally conductive filler containing a liquid metal (e.g., gallium, gallium/indium, gallium/indium/tin and/or mercury) into an unhardened matrix material (e.g., thermoplasts, thermosets, UV-curable materials, epoxies and solvent-bearing materials) and thereafter hardening the matrix material.
An English-language abstract of Japanese Patent Document JP 07-292251 appears to relate to curable thermally conductive electrically insulating magnesium oxide-containing silicone compositions.
International Patent Application No. PCT/US98/02531 provides a conductive, resin-based composition, which includes a resinous material, and a conductive filler. The conductive filler includes a first conductive filler and a second conductive filler. The particles of the first conductive filler are harder than those of the second conductive filler, when measured using the Mohs hardness scale. The composition is subjected to shear mixing forces which shearingly disperse the first and second conductive fillers throughout the resinous material in such a way that the particles of the second conductive filler occupy the interstitial voids within the network of first conductive filler particles contained in the resinous material and thereby enhance conductivity.
Silicone products of the type noted above (sometimes referred to as “MQ” resins) have been used to impart reinforcement properties to cured elastomers of heat-curable silicone compositions without increasing the viscosity of the composition, and while maintaining the clarity of the composition. MQ resins generally are copolymers of siloxanes formed from reactive trialkylsilyl (“M”) and tetraalkoxy silicate (“Q”) structures that can be prepared by either cohydrolyzing silanes containing M and Q units or by silylating inorganic silicates with trialkylsilyl containing silanes.
Vinyl- or hydride-containing silanes have been added during MQ resin preparation to yield MQ resins suitable for use in heat-cure silicone compositions. During heat cure, the vinyl groups on the MQ resin and the silicone fluid polymerize in a crosslinked network with the MQ resin incorporated in the network for reinforcement.
To date, commercially available thermally conductive compositions have suffered from a tendency to form crack-like voids in thin bonding applications, such as those common in the electronics industry—e.g., an electronic device (such as a silicon die) bonded to a heat sink (such as an aluminum heat sink). These crack-like voids (or microvoids) are thought to be caused by one or more of gas evaluation and/or shrinkage during cure, the cure temperature chosen, the type of conductive filler chosen, the nature, porosity and/or cleanliness of the substrate. In addition, commercially available silicone-based compositions have provided initial adhesion strength that could stand to be improved.
There, therefore, is an unfulfilled need for conductive, silicone-based compositions having superior conductivity characteristics without compromising the integrity of the mechanical properties of a cured reaction product, and more specifically demonstarting improved initial adhesion and reduced microvoiding.